feat: add support for fetching a greeting CTA as part of the update flow MONGOSH-1897

[nirinchev]

This adds support for fetching a greeting CTA via the versions json. The proposed design adds support for reading a cta field in the version objects:

 {
   "version": [
     {
       "version": "1.2.3",
       "downloads": [ ... ],
+      "cta": {
+        "chunks": [
+          { "text": "Fill out our survey ", "style": "bold" }
+          { "text": "https://mongodb.com/survey", "style": "mongosh:uri" }
+        ]
+      }
     }
   ]
 }

These are optional and nothing will be displayed if they're not present. If they are present and the version matches the current version, we'll format the chunks with their corresponding styles and display the message as part of the greeting.

Additionally, we add the following to the update-metadata.json:

 {
     "updateURL": "https://downloads.mongodb.com/compass/mongosh.json",
     "lastChecked": 1730881696340,
     "etag": "\"7f84f631518c4872e3e469c765d1eb03\"",
     "latestKnownMongoshVersion": "1.2.3",
+    "cta": {
+        "1.2.3-alpha.1": {
+           "chunks": [
+                { "text": "This is an alpha version! Use with caution!", "style": "bold" }
+            ]
+        },
+        "1.2.3": {
+           "chunks": [
+                { "text": "This is a stable version and very reliable!", "style": "bold" }
+            ]
+        },
+    }
 }

The greeting CTA for the current version based on package.json will be picked, but locally we're saving both the current and the latest version's CTAs to make it readily available once the user upgrades.

[nirinchev]

This is ready for review - I tested it with a custom S3 bucket and feel fairly confident that it works. The only remaining task is provisioning credentials for the downloads S3 bucket, adding them as a secret to GHA, and testing the workflow for updating the cta. Waiting on devprod there and I'll make sure to test it out before merging.

[nirinchev]

@addaleax and @gagik, pinging you for another review as I had to rework nontrivially how the CTA config is read, because I realized we also needed it when publishing new mongosh versions. Now it's a simple json with an attached json schema that is properly validated at publishing time. All the fun changes are in b1fb4f8 (#2254), but feel free to take another look at the whole thing as it's been a while 😄

I feel fairly confident this is now correct and ready for merging.

[nirinchev]

Evergreen failures point to issues that exist on main, so going to go ahead and merge this.